1. Field of the Invention
The present invention relates to a data transmission system for transmitting document or image data.
2. Description of the Prior Art
With recent developments in communication techniques, document or image data is transmitted through a public telecommunication line or a private communication line such as a coaxial cable or an optical fiber cable. Two methods are known for transmitting document data through a line; a method using character codes and a method using dot image data.
When document image data is transmitted using character codes, each character is transmitted with an 8- or 16-bit code which is obtained in accordance with a suitable standard such as JIS (JAPANESE INDUSTRIAL STANDARD) ASCII. In this case, only 8 or 16 bits are required for one character, so that a large amount of data may be transmitted at high speed. For this reason, this method is widely adopted in input/output of document data into or from a computer or transmission/ reception of document data prepared by a word processor.
In contrast to this, when document data or the like is transmitted in the form of dot image data, an image is generally divided into a dot matrix having 8.times.8 dots or 16.times.16 dots per 1 mm.sup.2. Image or document data is transmitted with binary or quantized signals representing white or black dots. In this case, even if a dot matrix having 8.times.8 dots per 1 mm.sup.2 is used, 64-bit data must be transmitted per 1 mm.sup.2 of the image, resulting in an extremely large amount of data to be transmitted. Although band compression for transmitting by encoding such a dot array may be utilized, it only reduces the amount of data to be transmitted to 1/10 the original amount. Furthermore, when band compression is performed, a complex algorithm must be used for compression or expansion of data, thus itself interfering with high speed data transmission. However, only predetermined characters or symbols may be transmitted with the first method mentioned above, whereas any characters or figures may be transmitted by this second method utilizing dot image data. Moreover, since signals obtained by reading a document or the like with an image sensor consist of dot image data, it is hard to convert them into character codes but is relatively easy to transmit them without conversion.
Thus, both these methods have respective advantages and disadvantages and are therefore widely adopted.
When communication is performed through a private line as in a local area network, lines are conventionally installed for individual items of equipment. However, installation of a line is costly. Therefore, it is uneconomical to separately install both a line for transmission of data from a computer or a word processor in the form of character codes, and a line for transmission of dot image data read from an original by a high-speed facsimile or digital copier. It is therefore desired that both data representing a dot image and data representing character codes be transmitted through a single line.
However, even if a signal line can allow transmission of data of these different formats, it does not allow communication between a computer or a word processor which can only process character code data and a high-speed facsimile system or the like which can only process dot image data. However, it is practically convenient if a document prepared by equipment which can only process character code data such as a computer or a word processor may be produced (e.g., printed) by equipment which can only process dot image data such as a high-speed facsimile system. Conversely, it is very convenient if an original image read by a facsimile system or the like may be entered into a computer through a communication line.
In view of this, a character generator for converting dot image data into character codes is conventionally incorporated into a printer which can only process dot image data, so that the printer may be able to receive and print either character code data or dot image data. If dot image data from a facsimile system or the like is to be converted into character codes, a character recognition apparatus at the receiver side recognizes characters corresponding to a received dot image and converts the dot image into character codes.
A character generator or a character recognition apparatus is generally large in size and is expensive. For this reason, it is impractical to connect such a character generator or character recognition apparatus to all the printers and computers connected to the line.
It is also impossible to directly connect a device which can only process compressed data with a device which can only process dot image data.